1. Field of the Invention
The invention relates to an at least partially percussive electrical hand-held power tool, in particular a chisel hammer or a pin hammer.
2. Description of the Prior Art
Usually the striking mechanism of percussive electrical hand-held power tools with a rear hand handle are formed as pneumatic striking mechanisms having a percussion piston movable at least in part along the length of a guide cylinder and which is actuated by a pneumatic spring which, in turn, is guided by an actuation means driven by an electrical motor. The plurality of mechanically actuated components for driving the striking means that actually apply an impact force on front end of the tool or on an anvil situated therebetween, substantially increases the passive mass of an electrical hand-held power tool.
According to U.S. Pat. No. 1,966,446, the striking mechanism in an electrical hand-held power tool is configured as an axially resonant vibrating rod oscillator, which is mounted in the middle on a vibration nodal point and is excited to vibrations of about 16 KHz using a piezo element supplied with an a.c. current. Such a configuration results, even with excitation in the ultrasound range in a necessary length, which is not suitable for an electrical hand-held power tool.
According to U.S. Pat. No. 3,681,627, an electrical hand-held power tool which is formed as an ultrasound drilling machine, is configured as a unit that includes a housing and a tool and is formed as an axially resonant vibrating rod oscillator excited by a piezo element, which is powered by a.c. to vibrations at 20 kHz. In this case, the annular piezo element is combined with the tool which, oriented opposite to the piezo element, vibrationally extends through the piezo element. By virtue of the respective alternatingly oriented, coaxial nesting of the housing, the piezo element and the tool, the overall structural length of the electrical hand-held power tool is reduced. In the case of this ultrasound drilling machine that is manually pressed by the user against a workpiece, the ultrasound vibrations of the tip of the tool cause only a sporadic lifting of same from the workpiece. A necessary, high-energy impact force is not triggered by the ultrasound vibration, especially when chiseling brittle materials like concrete or masonry.
According to U.S. Pat. No. 3,683,470, a percussive ultrasound—supported electrical hand-held power tool has an inherently pressure—biased piezo actor as the sole exciting means. An electrical hand-held power tool pressed by the user against a workpiece executes, as the result of excitation of the whole unit, a resonant low—frequency primary vibration of about 50 Hz, which is defined by the mass of An electrical hand-held power tool and the effective spring constant of the hand—arm system of the user. During approximately ⅓ of this primary vibration period, an ultrasound triggering of approximately 10 kHz, which is produced by the piezo actor, is superimposed on the dead center of the movement facing towards the workpiece. The necessary high-energy impact force when chiseling brittle materials like concrete or masonry is not generated by the ultrasound package.
According to EP 1060798, modern precopressed piezo actors are suitable for generation of high-energy axial longitudinal impact sound vibrations and at the same time for use as vibration sensors.
According to DE 19830415 A1, a percussive and rotating pulse striking mechanism for driving a probe through the ground operates with a high-energy piezo element. The piezo element expands when a voltage pulse is applied in fractions of a second by a defined magnitude. This abrupt expansion is used to effect a pulse on an anvil and for accelerating a percussion piston in the rearward direction against a spring biased against the housing. The percussion piston consequently receives a kinetic energy, which is stored in the spring and then is again transmitted to the percussion piston. The percussion piston then effects an impact force on the anvil via the piezo element. This impact force is detected electrically at the piezo element to adaptively trigger the next electrical activation of the piezo element, whereby the pulse striking mechanism is auto-excited. A spring, which is biased towards the rear of the housing and acts on the percussion piston, necessarily leads to strong housing vibrations, which must be subdued in electrical hand-held power tools, especially at the rear handle.